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Supercritical or transcriticalPower generation cycle
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Effect of P & T on the Rankine cycle
Effect of boiler P ; cycle ,
heat absorption T ,
moisture content in the turbine exit
turbine , erosion of turbine blade
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Several prototype pulverised combustion-based
supercritical steam cycle plants (using steam pressures
and temperatures of 240 bar and 590C)
due to various technical problems, these units were
found to be unreliable and uneconomic to operate. As aresult, most of these units were withdrawn from service in
the 1980s.
However, experiences with latest design of supercritical
plants are very encouraging.
Efficiencies up to about 44% or approaching 50%
http://www.cormix.info/pdf/Burns&McDonnell2001-3.pdf
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[H.W.] Supercritical Steam Power Plant with Reheat
In an effort to decentralize the national power distribution grid, the following
reheat-cycle, supercritical steam power plant (modeled after the Gavin Power
Plant in Cheshire, Ohio) has been proposed to service about 10,000
households in Athens, Ohio. It is to be placed close to the sewage plant on the
east side of Athens and cooled by water from the Hocking river.
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1. Neatly sketch the complete cycle on the P-h diagram below, indicating clearly all sixstations on the diagram.2. Assuming that both turbines are adiabatic, determine the combined power output of
the two turbines. The steam pipe at the inlet to the low pressure (LP) turbine has adiameter of 20 cm, and that at the outlet of the LP turbine leading to the condenserhas a diameter of 40 cm. Determine and discuss the effect of kinetic energy change onthe performance of the LP turbine.3. Assuming that the feedwater pump is adiabatic, and that the compressed liquidexperiences a change in temperature of 5C while passing through the pump,
determine the power required to drive the pump. Discuss the significance of this smallchange in temperature on the performance of the pump.4. Determine the total heat transfer to the boiler, including the reheat system.5. Assume that all the heat rejected from the condenser is absorbed by cooling waterfrom the Hocking river. To prevent thermal pollution the cooling water is not allowedto experience a temperature rise above 10C. If the steam leaves the condenser as
saturated liquid at 50C, determine the required minimum volumetric flow rate of thecooling water (cubic meters/minute).6. Determine the overall thermal efficiency L of this power plant. (Thermal efficiencyis defined as the net work done divided by the total heat supplied externally to theboiler and reheat system).
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Supercritical or transcriticalrefrigeration cycle
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Natural refrigerant
CFC, HCFC, HFC
J-T effect
Thermodynamic property
Transport propertyHistory
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0 50 100 150 200 250 300 350 400
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
6000
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
5500
6000
120 oC
140
oC
160
oC
10 0oC
80oC
60oC
40oC
20oC
0oC
C:/JKT/Scroll/R134a/FreonTotal/Total.opj/Graph-R134aPh
P-h diagram ofR134a
T=-20oC
Pressure,P[kPa]
Enthalpy, h[kJ/kg]
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-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
-60
-40
-20
0
20
40
60
80
100
120
140
160
180
200
220
-60
-40
-20
0
20
40
60
80
100
120
140
160
180
200
220
1400 kPa
600 kPa
C:/JKT/Scroll/R134a/FreonTotal/Total.opj/Graph-R134aTs
T-s diagram ofR134a
500
0kP
a
4500
kPa
4000k
Pa
3500
kPa
3000 kPa
2500 kPa
2000 kPa
1500 kPa
1000 kPa
700 kPa
500 kPa
300 kPa
200 kPa
P=100 kPa
Temperature,T[oC]
Entropy, s[kJ/kg K]
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Assessment of Carbon Dioxide as a Refrigerant
In the early part of this century, carbon dioxide (R-744) was widely used as a refrigerant.
Its popularity was based principally on its low toxicity, non-flammability, low cost and
universal availability. Other refrigerants, such as ammonia, sulfur dioxide and methylene
chloride could achieve much higher cycle efficiencies, but had other handicaps that
limited their application. The advent of chlorofluorocarbons (CFCs) in the 1930's
provided refrigerants that had low toxicities as well as high cycle efficiencies, removingmost of the incentives for choosing carbon dioxide its use thus dropped sharply.
Why consider R-744 now?
The ban on the use of CFCs and the phaseout of hydrochlorofluorocarbons (HCFCs)
have necessitated the search for new refrigerants and the reevaluation of old ones. The
reasons for carbon dioxide's original popularity, i.e., low toxicity, non-flammability, low
cost and universal availability, are still strong selling points.
However, two basic features still limit its acceptance: low cycle efficiency and high
operating pressures.
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Cycle Efficiency
R-744 has a low critical temperature of 31.1C (88.0F). If it is applied in a vapor
compression cycle whose heat rejection temperature is less than this, it will havetwo-phase condensation as well as evaporation, as do most other refrigerants.
However, the efficiency of the simple cycle is very low. For example, with an
evaporating temperature of -15C (5F) and a condensing temperature of 30C(86F), the refrigerating cycle coefficient of performance (COP) is only 2.81, ascompared to 4.77 for ammonia, 4.67 for R-22 and 4.41 for R-134a.
For heat rejection temperatures above 31.1C (88.0F), R-744 can still be used,but the high temperature, super-critical vapor does not condense at a single
temperature at a single temperature -- it changes gradually to a dense liquid as its
temperature is reduced. The refrigerating cycle efficiency remains low, however.
Cycle efficiency for R-744 can be improved in at least two ways.
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First, throttling losses can be reduced. As Lorentzen (Ref. 4) has pointed
out, a large part of the cycle inefficiency for R-744 is the energy (work) lost
in the throttling process. By employing an ideal (isentropic) expander torecover work, compressor work is reduced and the cycle COP can be
raised by over 50%. There is a good opportunity for using real (non-ideal)
expanders. Of course, a drawback is the added cost of such devices.
Second, liquid line-suction line heat exchangers can be employed at an
added cost. Pettersen (Ref. 4) used such a device to lower the liquidtemperature before throttling, raising the evaporator capacity and lowering
the throttling losses.
The net cycle COP improvements appear to be less than 5%.
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Operating Pressures
Evaporating pressures for typical air conditioning duty using carbon dioxide are about
3,400 to 4,800 kPa (490 to 700 psia) while high-side pressures are about 8,300 to
13,800 kPa (1,200 to 2,000 psia).
These pressures are about five times higher than with conventional refrigerants. This
presents obvious problems of providing thicker walls for piping, heat exchangers,
receivers and compressor shells. On the other hand, the higher fluid densities lead to
lower velocities and lower pressure drops. The higher densities can also lead to more
compact heat exchangers.
Perhaps the most serious challenge is the design of new compressors for these high
pressures. On the positive side, it has been claimed that pressure ratios for R-744 are
lower than those for other refrigerants and therefore will improve compression efficiency
(Ref. 5, Ref. 7 and Ref. 8).
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In summary,
carbon dioxide (R-744) has several attractive attributes: low toxicity,
nonflammability, low cost and wide availability. However, its inherently low
refrigerating cycle efficiency and high operating pressures will remain
serious challenges to its use in unitary heating and cooling products in the
near term.
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Compressors aren't the only part of a cooling system where
innovation can be found. Improving the heat transfer
characteristics of heat exchangers can also boost a cooling
system's efficiency. A good example is the PFT (parallel flow)
technology developed by Modine Manufacturing Co., Racine,
Wis.
Conventional heat exchangers use round copper tubing with
mechanically bonded aluminum fins. By contrast, PF heat
exchangers are all-aluminum brazed construction, with the
fins metallurgically bonded. More importantly, in a PF heat
exchanger, the tubing has a flattened shape, and its interior is
sectioned into a series of multiple, parallel flow,
microchannels that carry the refrigerant or working fluid.
The microchannels in the heat exchanger provide better heat
transfer on the refrigerant side.
Efficiency gains also found in heat exchangers
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References:
1. ASHRAE Handbook of Fundamentals, 1993, p.167.2. S. Klein and F. Alvarado, Engineering Equation Solver (EES), Version 4.259W,
Fchart Software, Middleton, WI, 1996.3. G. Lorentzen and J. Pettersen, "A New, Efficient and Environmentally Benign Systemfor Car Air Conditioning", International Journal of Refrigeration, 1993, Vol. 16, No.1,pp. 4-11.4. G. Lorentzen, "Revival of Carbon Dioxide as a Refrigerant", H & V Engineer, Vol.66, No. 721, 1994, pp. 9-14.
5. D. Robinson and E. Groll, "Using Carbon Dioxide in a Transcritical VaporCompression Refrigeration Cycle", Sixth International Refrigeration Conference atPurdue, 25 July 1996.6. J. Pettersen, "An Efficient New Automobile Air-Conditioning System Based on CO2Vapor Compression", ASHRAE Transactions, 1994, Vol. 100, Part 2, pp. 657-665.7. G. Lorentzen, "The Use of Natural Refrigerants: A Complete Solution to the
CFC/HCFC Predicament", International Journal ofR
efrigeration, 1995, Vol. 18, No.1,pp. 190-197.8. J. Koehler, M. Sonnekalb, H. Kaiser and W. Koecher, "Carbon Dioxide as aRefrigerant for Vehicle Air-Conditioning with Application to Bus Air- Conditioning",1995 International CFC and Halon Alternatives Conference, October 1995, pp. 376-385.